Implement modular compilation

test/codegen/expect/js/src/varargs.js

-dart_library.library('js/src/varargs', null, /* Imports */[
-  'dart/_runtime',
-  'dart/core'
-], /* Lazy imports */[
-], function(exports, dart, core) {
-  'use strict';
-  let dartx = dart.dartx;
-  class _Rest extends core.Object {
-    _Rest() {
-    }
-  }
-  dart.setSignature(_Rest, {
-    constructors: () => ({_Rest: [_Rest, []]})
-  });
-  const rest = dart.const(new _Rest());
-  function spread(args) {
-    dart.throw(new core.StateError('The spread function cannot be called, ' + 'it should be compiled away.'));
-  }
-  dart.fn(spread);
-  // Exports:
-  exports.rest = rest;
-  exports.spread = spread;
-});